efi_loader: increase eventlog buffer size

TCG PC Client PFP spec says "The Log Area Minimum Length
for the TCG event log MUST be at least 64KB." in ACPI chapter.
This commit increase the buffer size to 64KB.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>