gpio: pca953x: make the register access by GPIO bank

Until now the pca953x driver accessed all the bank of a given register
in a single command using only a 32 bits variable. New expanders from
the pca53x family come with 40 GPIOs which no more fit in a 32
variable. This patch make access to the registers more generic by
relying on an array of u8 variables. This fits exactly the way the
registers are represented in the hardware.

It also adds helpers to access to a single register of a bank instead
of reading or writing all the banks for a given register.
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Tested-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>