ARM: shmobile: r8a7779: Reserve boot area when SMP is enabled

CPU core bringup on R-Car H1 uses the ARM Reset Vector Address Register
(AVECR) 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 AVECR 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/924961188abdf2adb52167edb7ed848a6efcca39.1693409184.git.geert+renesas@glider.be