ram: stm32mp1: Add STM32MP13xx support

ram: stm32mp1: Add STM32MP13xx support

Add support for configuring DRAM controller on STM32MP13xx SoC.
The DRAM controller is basically identical to the DWC controller
on STM32MP15xx SoC, except the bus width is reduced from 32bit to
16bit and a few registers and bits are therefore not present.

Handle the difference by factoring these parts out. Use IS_ENABLE()
as much as possible to assure code which is not enabled on builds
for a single SoC gets compiled out. Handle the different offset of
RCC_DDRITFCR register and missing DDRC2 clock the same way.

Signed-off-by: Marek Vasut <marek.vasut@mailbox.org>