ARM: shmobile: sh73a0: Reserve boot area when SMP is enabled

CPU core bringup on SH-Mobile AG5 uses the SYS Boot Address (SBAR) and
Address Translation Area (APARMBAREA) registers to specify the base
address and size of the boot area of the System CPU.  With this enabled,
when the System CPU accesses a physical address in the range from zero
up to the configured size, the top address bits are replaced by those
specified in the SBAR register.  Hence any device residing in this low
part of physical address space cannot be accessed.

Prevent conflicts by reserving this memory region using
request_mem_region().

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20eb4e7fca9c41a34500fc5984602b41006b4575.1693409184.git.geert+renesas@glider.be