runtime/volatile: add GetBits

[firelizzard18]

Adds another convenience method for registers to read multiple bits at an offset. This change will allow cleaner code:

// instead of
(nxp.MCG.S.Get() & nxp.MCG_S_CLKST_Msk) != (2 << nxp.MCG_S_CLKST_Pos)

// use
nxp.MCG.S.GetBits(nxp.MCG_S_CLKST_11, nxp.MCG_S_CLKST_Pos) != 2

[deadprogram]

Isn't this mostly the same as https://github.com/tinygo-org/tinygo/blob/dev/src/runtime/volatile/register.go#L56

nxp.MCG.S.HasBits(nxp.MCG_S_CLKST_11 << nxp.MCG_S_CLKST_Pos)

Also, all of the related functions in volatile return a bool.

[firelizzard18]

HasBits returns whether any bit is set at the given positions, which is why it returns a boolean, whereas GetBits extracts the value of the specified bits, regardless of whether they are all set or not, which is why it returns a uintN.

MCG.S	HasBits	GetBits
xxxx00xx	false	00
xxxx01xx	true	01
xxxx10xx	true	10
xxxx11xx	true	11

• nxp.MCG.S.HasBits(nxp.MCG_S_CLKST_11 << nxp.MCG_S_CLKST_Pos) returns true if MCG.S is not xxxx00xx
• nxp.MCG.S.GetBits(nxp.MCG_S_CLKST_11, nxp.MCG_S_CLKST_Pos) returns AB given MCG.S is xxxxABxx
• nxp.MCG.S.GetBits(nxp.MCG_S_CLKST_11, nxp.MCG_S_CLKST_Pos) != 2 returns true only if MCG.S is xxxx10xx

[aykevl]

I think there is a better way to do this: by treating these struct fields as bitfields (which they are) and auto generating getters/setters for these bitfields in gen-device-svd. At the moment, HasBits is already much too easy to misuse.

Compare this:

// current code
(nxp.MCG.S.Get() & nxp.MCG_S_CLKST_Msk) != (2 << nxp.MCG_S_CLKST_Pos)

// this PR
nxp.MCG.S.GetBits(nxp.MCG_S_CLKST_11, nxp.MCG_S_CLKST_Pos) != 2

// my proposal
nxp.MCG.S.GetCLKST() != 2

This would mean that the struct would look more like this (all autogenerated from the SVD file):

type MCG_Type struct {
    // ... other registers
    S MCG_S_Type
    // ... other registers
}

type MCG_S_Type struct {
    volatile.Register8
}

//go:inline
func (field *MCG_S_Type) GetCLKST() uint8 {
    return (field.Register8.Get() & nxp.MCG_S_CLKST_Msk) >> nxp.MCG_S_CLKST_Pos
}

Other options: adding setters, treating single-bit fields as booleans (returning a bool instead of an integer), ...
Eventually we could hopefully replace and remove HasBits/SetBits/ReplaceBits entirely.