mfd: stmpe: Rework registers access

mfd: stmpe: Rework registers access

this update allows to use registers map as following :
regs[reg_index + offset] instead of
regs[reg_index] + offset

This makes code clearer and will facilitate the addition of STMPE1600
on which LSB and MSB registers are respectively located at addr and addr + 1.
Despite for all others STMPE variant, LSB and MSB registers are respectively
located in reverse order at addr + 1 and addr.

For variant which have 3 registers's bank, we use LSB,CSB and MSB indexes
which contains respectively LSB (or LOW), CSB (or MID) and MSB (or HIGH)
register addresses (STMPE1801/STMPE24xx).
For variant which have 2 registers's bank, we use LSB and CSB indexes only.
In this case the CSB index contains the MSB regs address (STMPE 1601).

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>