mtd: rawnand: fscm: Avoid collision on PC def when compiling for MIPS

We want to allow this driver to be selected when COMPILE_TEST=y, this
means the driver can be compiled for any arch, including MIPS. When
compiling this driver for MIPS, we end up with a collision on the 'PC'
macro definition (also defined in arch/mips/include/asm/ptrace.h).

Prefix the fsmc one with FSMC_ to avoid this problem.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>