qemu-arm: round down memory to multiple of 2MB for LPAE

qemu-arm: round down memory to multiple of 2MB for LPAE

QEMU's -m option can take fractional megabyte values,
and lowest granularity seems to be 0x2000.
For example, run qemu with amount of memory set to 100005k (0x61A9400):

$ qemu-system-arm -machine virt -cpu cortex-a15 -m 100005k \
-bios denx/u-boot.bin -nographic

=> fdt addr $fdt_addr
=> fdt print /memory@40000000
memory@40000000 {
        reg = <0x00000000 0x40000000 0x00000000 0x061aa000>;
        device_type = "memory";
};

When LPAE is enabled, 1:1 mapping is created using 2 MB blocks.
In case amount of memory provided to QEMU is not multiple
of 2 MB, hang occurs during MMU initialization.

How to reproduce:
qemu-system-arm -machine virt -m 1058 -nographic -bios u-boot.bin - boots
qemu-system-arm -machine virt -m 1057 -nographic -bios u-boot.bin - hangs

DRAM:  1 GiB
initcall: 60011df8
initcall: 60011904
New Stack Pointer is: 80fffe90
initcall: 60011a20
initcall: 60011bcc
initcall: 60011bd4
initcall: 600119b4
Relocation Offset is: 22042000
Relocating to 82042000, new gd at 81001ed0, sp at 80fffe90
initcall: 60011b8c
initcall: 82053ea0
initcall: 82053ea8
initcall: 60012040 (relocated to 82054040)
dram_bank_mmu_setup: bank: 0
- hang here during mmu init -

This patches rounds down to the nearest multiple of 2MB when
CONFIG_ARMV7_LPAE=y.

Fixes: 3fa914af82("arm: qemu: implement enable_caches()")
Signed-off-by: Igor Opaniuk <igor.opaniuk@foundries.io>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>