release/riscv: tweak GENERICSD partition placement

Increase EFI partition size to begin rootfs at 64mb. I believe this was
my original intention. I have a microSD card with 8mb block size which
emits an advisory in verbose dmesg about the misaligned partition.

MFC after:	1 week
Sponsored by:	The FreeBSD Foundation

release/riscv/GENERICSD.conf

@@ -3,7 +3,7 @@
 EMBEDDED_TARGET_ARCH="riscv64"
 EMBEDDED_TARGET="riscv"
 EMBEDDEDBUILD=1
-FAT_SIZE="54m -b 8m"
+FAT_SIZE="56m -b 8m"
 FAT_TYPE="16"
 IMAGE_SIZE="6144M"
 KERNEL="GENERIC"