spl: increase SPL_SYS_MALLOC_SIZE when using BIOSEMU on RISC-V

If BIOSEMU is compiled for RISC-V (SiFive Unmatched board) and the function
dm_pci_run_vga_bios() is executed, U-Boot stops with error message saying
that the SPL malloc pool is too small. So increase the default pool size
when both BIOSEMU and RISCV parameters are set.

Signed-off-by: Yuri Zaporozhets <yuriz@qrv-systems.net>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
diff --git a/common/spl/Kconfig b/common/spl/Kconfig
index 045fcac..240543c 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -462,6 +462,7 @@
 config SPL_SYS_MALLOC_SIZE
 	hex "Size of the SPL malloc pool"
 	depends on SPL_SYS_MALLOC
+	default 0x180000 if BIOSEMU && RISCV
 	default 0x100000
 
 config SPL_READ_ONLY