serial: max310x: Move all SPI-dependent stuff in one place

Patch move all SPI-dependent stuff in one place. This include move
PM-function definitions out of CONFIG_SPI_MASTER and move regmap
initialization out of common probe into CONFIG_SPI_MASTER which
may help to add I2C support in the future for this driver.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>