arm: Add more flash and ram size options to i.MX RT MPU config

Adds 8 MB and 64 MB flash size options, which correspond to the two
external flashes (qspi flash and hyperflash) on the mimxrt1050_evk
board. Adds a 32 MB SRAM size option, which corresponds to the external
sdram on the board.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>