mailbox: mchp-ipc-sbi: Fix COMPILE_TEST build error

If COMPILE_TEST is y but RISCV_SBI is n, build fails:

drivers/mailbox/mailbox-mchp-ipc-sbi.c: In function 'mchp_ipc_sbi_chan_send':
drivers/mailbox/mailbox-mchp-ipc-sbi.c:119:23: error: storage size of 'ret' isn't known
	struct sbiret ret;
	              ^~~
  CC      drivers/nvmem/lpc18xx_otp.o
drivers/mailbox/mailbox-mchp-ipc-sbi.c:121:15: error: implicit declaration of function 'sbi_ecall' [-Werror=implicit-function-declaration]
	ret = sbi_ecall(SBI_EXT_MICROCHIP_TECHNOLOGY, command, channel,
	      ^~~~~~~~~

move COMPILE_TEST to ARCH_MICROCHIP dependency as other drivers.

Fixes: e4b1d67e ("mailbox: add Microchip IPC support")
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>